1. Field of the Invention
This invention relates to apparatus for identifying and characterizing substances using Raman spectroscopy, which provides a non-contact and non-invasive technique for investigation and analysis of chemical substances.
2. Description of the Prior Art
Raman spectroscopy is widely used in the scientific, commercial and public safety areas.
Technological advances are making it possible to increase the range of applications using Raman spectroscopy, through reductions in costs and size of the equipment. Portable units have become available for field uses, such as on-site identification of potentially hazardous materials.
In applications of Raman spectroscopy, it is generally desirable to bring an optical probe to a position adjacent a specimen. This can be a problem in view of the potentially hazardous materials which are to be analyzed, including explosives, chemical agents, toxic industrial chemicals, and the like. In some applications, it is required, for safety reasons, that delivery of laser light to a specimen under test, and a collection of Raman signal from the specimen, be done at a location remote from the Raman spectrometer hardware. Optical fiber, which can serve as a conduit for laser light and Raman signal light, is a good medium for achieving this. However, there are some problems in the use of optical fibers and probes for Raman spectroscopy.
Firstly, the distal end of the probe can become contaminated during data collection and it is often desirable, and at times necessary, to replace the probe head, which is very costly, if it can be done at all. Accordingly, there is a need for a relatively inexpensive optical fiber assembly, including optical fiber and a probe head, which can be disconnected from the spectrometer and replaced with another optical fiber assembly.
Secondly, propagation of a high power laser light within an optical fiber generates unwanted Raman signal from the optical fiber material itself which adds to the Raman signal collected from a specimen and, in many cases, is difficult to distinguish from the specimen signal and difficult to subtract from the Raman signal generated from the specimen under test.
Accordingly, there is a further need for means for preventing Raman signals generated by the material of an excitation fiber of an optical fiber assembly from reaching the specimen under test, such that only the excitation laser signal reaches the specimen and the Raman signals received from the specimen and conducted to a spectrometer by way of a collection fiber of the optical fiber assembly are from the specimen only and not from the excitation fiber.
Thirdly, the laser light exiting the distal end of the excitation fiber diverges immediately and does so until the excitation light reaches the specimen under test. Thus, the portion of the excitation light which reaches the specimen reduces, increasingly, as the distance between the excitation fiber and the specimen increases.
There is accordingly a still further need for means to focus the light from the excitation fiber onto a small area of the specimen. Similarly, Raman signals reflected off the specimen diverge outwardly from the specimen with only a small portion of the reflected light reaching an end of a collection fiber. Accordingly, there is also a need for means to focus Raman light from a specimen onto the small area of an end of a collection fiber.